Increased production of an unknown steroid with mineralocorticoid properties is believed to be etiologic in a fraction of patients with Low Renin hypertension. We have designed and recently improved upon a mineralocorticoid radioreceptor assay which we are currently using to follow the purification of mineralocorticoids from urine of such patients. For this assay we are adding RU-26988, a newly developed "pure" glucocorticoid, to block the glucocorticoid receptor leaving only the mineralocorticoid receptor to be bound by tritiated aldosterone. We have shown that aldosterone binds to two receptor systems in rat kidney cytosol, Type I (mineralocorticoid receptor) with a Kd of 5 x 10 to the minus 9th power M and Type II (glucocorticoid receptor) with a Kd of 4 x 10 to the minus 8th power; however, when using kidney slices the Kd of Type I receptors is one order of magnitude lower (5 x 10 to the minus 10th power M) indicating that when cytosol is used the glucocorticoid receptor somehow interferes. The addition of RU-26988 to block the glucocorticoid receptor uncovers the mineralocorticoid receptor. Scatchard analysis shows a single receptor with a Kd of 5 x 10 to the minus 10th power M. We are using this radioreceptor assay to monitor the purification of several unknown steroids with radioreceptor activity in the urine of patients with Low Renin Hypertension. In the next year we expect to isolate and identify these compounds using gas chromatography-mass spectrometry and Nuclear Magnetic Resonance.